They will fit, but the front springs will be too soft for the heavier engine in you 550. You can look up part #'s at Eibach's website and you'll see they have one kit for 6 cyl engines and a different kit (with higher rate springs for the front) for the v8 cars.

This is correct. I would look into ST coilovers before considering springs. These cars are heavy and the coilover can provide better ride quality over the springs. I am personally running KW Street Comforts on my 545i.